The present invention relates to a method and apparatus for controlling a lifting magnet of a materials handling machine. It finds particular application in conjunction with lifting magnets used on cranes and other prime movers in the steel and scrap metal industries.
Lifting magnets are commonly used in the materials handling industry to lift and move magnetic materials. For example, in the steel industry, lifting magnets are used to move intermediate products and finished goods. Also, in the scrap metal industry, lifting magnets are commonly attached to cranes and other prime movers and used to load, unload, and otherwise move scrap steel and other ferrous metals.
While lifting magnets have been in common use for many years, the systems used to control these lifting magnets remain relatively primitive. Known control systems operate to selectively open and close contacts that, when closed, complete a circuit between a suitable source of DC electrical power and the lifting magnet. The source of DC power is generally at least 230 volts, and during certain lifting stages, the voltage can reach approximately 275 volts. Additionally, when the polarity of the voltage across the magnet is briefly reversed as is required to "push" a load of metal off of the magnet, voltages commonly reach 500-1000 volts. Thus, opening and closing the contacts during these conditions, to break or complete the magnet circuit, naturally results in arcing across the tips of the contacts and the creation of voltage spikes in the magnet control system.
Arcing between the contacts of known magnet controllers causes burning and wear which eventually leads to the need to replace the contacts. The large variation in voltage also eventually wears out the generator (the typical source for the DC power), the magnet and associated insulation, as well as the cables used to connect the magnet to the generator. To withstand the large voltages and voltage spikes, the magnet, cables, and the control system contacts and other components must be constructed of more expensive materials and must also be made larger in size.
Also, with known magnet control systems, the control system must be matched to the particular magnet being used. For example, the contacts and associated circuitry in a known magnet controller for a 93 inch diameter, 40 kilowatt (kW) magnet must be able to pass approximately 175 Amperes of current and also withstand very large voltage spikes. Such a controller would not be effective when used in conjunction with a 30 inch diameter, 5 kW magnet that draws only 20 Amperes of current. Of course, the components used in a controller for the smaller magnet would not be able to withstand the electrical current and voltage spikes associated with the larger magnet. Thus, with known systems, an operator of a scrap yard or other facility needs to restrict the use of different magnets on the various cranes and other prime movers or must switch the entire control system of the prime mover accordingly. For example, certain known magnet controllers are available in seven different capacities and each is unusable with magnets outside of its operational range. Therefore, a facility using different size magnets must also purchase and maintain a magnet controller suitable for use with each magnet.
Known lifting magnet control systems are not "user-friendly." These control systems do not provide the operator of the magnet with sufficient information regarding the status of the magnet and the magnet control system. For example, known systems do not inform the operator if there exists an unwanted ground in the magnet circuit. Such a ground can damage the magnet or its controller and also adversely affect the operation both the magnet and controller, resulting in dropped loads or other malfunctions. A ground to the chassis of the prime mover can also damage the electronics of the prime mover which are preferably completely isolated from the magnet circuit but which are often grounded to the machine chassis. An unwanted ground in the magnet circuit is also potentially harmful to the generator supplying power to the circuit.
Likewise, known magnet controllers do not monitor the "duty cycle" of the magnet. Duty cycle is the percentage of time that the magnet is energized or "turned on" relative to its total time in operation for a given period of time. Thus, to move a load of steel, an operator may have to energize the magnet 60% of the time, with the remainder of the time being accounted for by the time required to maneuver the magnet and its prime mover, as well as the time when the magnet is deenergized or "turned off" to drop a load. Modern magnets can withstand a 75% duty cycle. If this maximum duty cycle is exceeded, the magnet will be damaged. However, with known magnet control systems, operators are unable to effectively monitor duty cycle and known controllers do not inform the operator if the maximum duty cycle is being exceeded.
Known systems also do not monitor the condition of the generator that supplies DC electrical power to the magnet circuit. If the magnet is being heavily used, it is possible for the generator to overheat. If an operator is unaware of a generator overheating problem, the generator will be damaged. Thus, it would be desirable to provide a magnet control system that continuously monitors the condition of the generator and informs the operator if the generator begins to overheat.
Further, known system do not allow the operator to adjust the "drop time"--the amount of time a reverse voltage is applied to the magnet to reverse its polarity--without assistance or without leaving the operator's cab. Known systems require that this adjustment of drop time be made at the controller itself, which is usually accessible underneath or at the rear of the crane or other prime mover. This is dangerous and difficult, especially due to the fact that test lifts and drops must be made during the adjustment operation. Thus, either the operator of the prime mover machine must repeatedly exit the operator's cab and adjust the drop time or a second person must adjust the drop time in response to commands from the operator. This second person could easily be electrically shocked or otherwise injured should the operator unexpectedly activate the lifting magnet or the prime mover machine itself.
Another drawback associated with known magnet control systems relates to the fact that the generator providing DC power to the magnet is generally driven through a belt-drive connection or using a hydraulic motor which is powered by a hydraulic pump connected to the main engine or an auxiliary engine of the prime mover. Thus, with known systems, an increase or decrease in revolutions per minute (rpm) in the engine driving the generator results in a corresponding increase or decrease in the rpm of the generator armature. This consequently results in an increase or decrease in the DC power output from the generator. While a certain amount of over-voltage from an increase in engine rpm is acceptable, a severe under-voltage, as might occur upon the driving engine becoming "bogged down" or otherwise slowed, can result in a severe drop in generator output to the magnet. If insufficient power is supplied to the lifting magnet, its load could be accidentally dropped. Attempts to utilize conventional voltage regulators to overcome these voltage variations have not been successful. Specifically, conventional voltage regulators cannot withstand the large voltage spikes associated with known magnet controllers.
In order to drop a load from an electro-magnetic lifting magnet, the forward voltage applied to the magnet must be turned off, and a reverse voltage must be applied to drive the current down to 0 and then momentarily reverse the current to correspondingly reverse the magnetic field of the magnet so that the load is repelled or "pushed" from the magnet. Merely interrupting the forward voltage to the magnet without applying a reverse voltage would cause at least part of the load to be held on the magnet due to the magnet's residual magnetism.
Heretofore, lifting magnet controllers have relied solely on conventional mechanical or electronic timers or other timing devices such as relays and the like to control the onset and duration of magnet "reverse current" used to reverse the polarity of the lifting magnet to "push" the load off of the magnet during a drop cycle. However, use of these timing devices has been found to be inefficient, inconvenient, and often results in poor magnet drop characteristics. These deficiencies result from a variety of factors. For instance, these prior systems do not dynamically and automatically account for variations in lifting magnet size, lifting magnet temperature, load size, load type, and other such variations which alter the drop characteristics of the magnet. Therefore, for example, even if the magnet controller is manually adjusted so that the magnet properly picks and drops a particular type and size load, any variation in load size and type, and/or any large variation in magnet temperature, will cause the drop characteristics of the magnet to change. This, then, requires the operator or a co-worker to access the controller and manually vary the timer setting as needed. Obviously, such manual adjustment of the reverse current timer is inefficient and inconvenient. Furthermore, proper manual adjustment of the reverse current timer in prior controllers depends upon the skill of the operator in recognizing that an adjustment is needed and in actually adjusting the timer for optimal load-drop characteristics. Often, an operator either does not recognize an adjustment to the drop timer is needed, does not have the time or ability to adjust the timer, or improperly adjusts the timer mechanism. The foregoing deficiencies are especially critical given that even a 10.sup.th of a second variation in the duration of the reverse current through the magnet can significantly affect the magnet drop performance.
Because prior magnet controllers do not accurately control the onset and duration of the reverse current in a drop cycle, manufacturers of these prior controllers have designed the controllers to use only a portion of the forward or "lifting" voltage in the drop cycle. By applying only a reduced reverse voltage to the magnet to develop the reverse current, these prior controllers are attempting to accommodate the above-noted inaccuracies in the reverse current timer. That is to say, these prior controllers do not use a reverse or drop voltage that is equal to the forward or lift voltage because errors in the timing of the reverse current could cause the load to be initially repelled and then re-attracted to the magnet during the drop cycle. After termination of the reverse current, the load would merely dribble off the magnet. Use of a reduced drop voltage in prior controllers is intended to compensate for overly long drop cycles due to timer errors. However, the use of a reduced drop voltage also increases the time required to push a load from the magnet which leads to further inefficiencies.